Various cold cathodes have been developed through microfabrication technology or thin-film formation technology. Applications of such cold cathodes for electron-beam-generating apparatuses including flat panel displays, discharge tubes, lamps, vacuum micro/nano devices, and the like has been studied. The realization of electronic devices, electronic instruments, and the like using properties of the cold cathodes, which is difficult in case of using solid state semiconductor devices, has been expected. Obtainment of a high current with low voltage is essential for the realization of such application. Accordingly, the applications of cold cathodes have been studied and developed from both material and structural viewpoints.
From the material viewpoint, materials with low work functions are promising, so that oxides such as zirconium oxide, nitrides such as titanium nitride and aluminum nitride, and carbon-based materials such as diamonds and diamond-like carbon are subjects of search and development. Meanwhile, formation of a sharp needle or a cone-shape structure is required for a cold cathode material such as conventionally known molybdenum or tungsten in order to efficiently obtain a high current with low voltage. Production with the use of nanotechnology that has recently remarkably progressed is also employed.
Diamond has a band gap that is as wide as 5.5 eV. However, the electron affinity on the surface is negative. Thus, diamond has been suggested as a good cold cathode material (see JP Patent Publication (Kokai) No. 2002-15658 A). Furthermore, aluminum nitride and boron nitride (which also have negative electron affinity) are similarly expected to be good cold cathode materials (see JP Patent Publication (Kokai) No. 2002-352694 A). Among these materials having such negative electron affinity, diamond is the most likely candidate, since diamond is excellent in terms of material synthesis and controllability, and because nanoprocessing technology for diamond has also been developed (see JP Patent Publication (Kokai) No. 10-312735 A (1998)). Also in terms of other physical properties including a high degree of hardness, thermal conductivity, and chemical stability, diamond is the best candidate as an electron emission material since diamond is a covalently-bound monoatomic material.
The negative electron affinity of diamond appears when a diamond surface is terminated with hydrogen, titanium, nickel, or the like. It has been reported that electron emission is observed with voltage lower than that of conventional metals or semiconductor materials through the use of such surface (see P. K. Baumann et al, Surface Science 409 (1998) 320). To use such surface feature, exciting or injecting electrons into a conduction band are necessary. Operation with low voltage is confirmed through addition of nitrogen or phosphorus, which is an impurity as a donor with a high concentration (see K. Okano et al, Nature 381 (1996) 140). However, electron emission that had actually elicited the feature of negative electron affinity was observed when the surface was terminated with cesium (see M. W. Geis et al, Applied Physics Letters 67 (1995) 1328). The use of caesium, which is handled with difficulty in terms of industrial application, is also problematic from an environmental viewpoint. Caesium has also high reactivity, so that the long-term stability thereof cannot be realized. Furthermore, negative electron affinity is also observed on a hydrogen-terminated surface. Specifically, the termination structure is stable in the air; however, it requires operation in an ultrahigh vacuum or hydrogen atmosphere from the viewpoint of stability of electron beam source operation. Such hydrogen-terminated surface has excellent basic characteristics, but is still problematic in terms of device operation.